Solve for $r$ : $r - 15 = -23$
Answer: Add $15$ to both sides: $ \begin{eqnarray} \\ r - 15 &=& -23 \\ \\ {+15} && {+15} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{-15 + r} &\hphantom{=}& \hphantom{-23} \\ r &=& -23 {+ 15} \\ \end{eqnarray} $ Simplify. $ r = -8$